U.S. patent application number 12/480374 was filed with the patent office on 2009-10-01 for method for damping edgewise oscillations in one or more blades of a wind turbine, an active stall controlled wind turbine and use hereof.
Invention is credited to Thomas Steiniche Bjertrup Nielsen, Bo Juul Pedersen, Christopher John Spruce.
Application Number | 20090246020 12/480374 |
Document ID | / |
Family ID | 39492648 |
Filed Date | 2009-10-01 |
United States Patent
Application |
20090246020 |
Kind Code |
A1 |
Nielsen; Thomas Steiniche Bjertrup
; et al. |
October 1, 2009 |
Method For Damping Edgewise Oscillations In One Or More Blades Of A
Wind Turbine, An Active Stall Controlled Wind Turbine And Use
Hereof
Abstract
A method for damping edgewise oscillations in one or more blades
of a wind turbine includes the steps of detecting if one or more of
the blades oscillates edgewise during operation of the wind
turbine, and substantially cyclically generating a pitch angle
difference between at least two of the blades. Further provided is
an active stall controlled wind turbine and use hereof.
Inventors: |
Nielsen; Thomas Steiniche
Bjertrup; (Randers, DK) ; Pedersen; Bo Juul;
(Hadsten, DK) ; Spruce; Christopher John;
(Leatherhead, GB) |
Correspondence
Address: |
ST. ONGE STEWARD JOHNSTON & REENS, LLC
986 BEDFORD STREET
STAMFORD
CT
06905-5619
US
|
Family ID: |
39492648 |
Appl. No.: |
12/480374 |
Filed: |
June 8, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/DK2007/000520 |
Nov 26, 2007 |
|
|
|
12480374 |
|
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Current U.S.
Class: |
416/1 ;
416/31 |
Current CPC
Class: |
F03D 7/024 20130101;
F05B 2270/334 20130101; Y02E 10/723 20130101; F05B 2260/96
20130101; F03D 7/0296 20130101; F03D 13/35 20160501; F05B 2270/807
20130101; F03D 7/0224 20130101; Y02E 10/72 20130101; F03D 7/042
20130101; Y10S 416/50 20130101 |
Class at
Publication: |
416/1 ;
416/31 |
International
Class: |
F03D 7/02 20060101
F03D007/02 |
Claims
1. A method for damping edgewise oscillations in one or more blades
of a wind turbine, said method comprising the steps of detecting if
one or more of said blades oscillates edgewise during operation of
said wind turbine, and substantially cyclically generating a pitch
angle difference between at least two of said blades, characterized
in that said pitch angle difference between at least two of said
blades generates an asymmetric load situation on a hub centre of a
rotor to which said blades are attached, which asymmetric load
situation substantially counter-phases an asymmetric load situation
generated by said edgewise oscillations.
2. The method according to claim 1, wherein said pitch angle
difference is only generated if size of said edgewise oscillations
is above a predefined level in one or more of said blades.
3. The method according to claim 1, wherein a size of said pitch
angle difference is generated substantially directly proportional
to a size of said edgewise oscillations.
4. The method according to claim 1, wherein a sum of the thrust
provided by said blades are being substantially maintained when
generating said pitch angle difference between at least two of said
blades.
5. The method according to claim 1, wherein said pitch angle
difference is generated by offsetting a pitch angle of at least a
first blade in a first direction and offsetting a pitch angle of at
least one further blade in an opposite direction of said first
direction.
6. The method according to claim 5, wherein said offset is
generated in addition to a normal pitch angle algorithm controlling
the pitch angle of said blades in relation to normal wind turbine
control parameters.
7. The method according to claim 1, wherein said edgewise
oscillations are detected by use of one or more oscillation sensors
placed in one or more of said blades.
8. The method according to claim 1, wherein said edgewise
oscillations are detected by use of one or more oscillation sensors
placed in or at a rotational axis of a rotor on which said blades
are amounted.
9. The method according to claim 7, wherein said oscillation
sensors are one or more accelerometers.
10. The method according to claim 1, wherein a gain of a control
algorithm controlling a relation between said pitch angle
difference and said edgewise oscillations is increased if a size of
said edgewise oscillations rises above a predefined level.
11. The method according to claim 1, wherein a gain of a control
algorithm is controlling a relation between time derivatives of
said pitch angle difference and said edgewise oscillations.
12. An active stall controlled wind turbine comprising control
means for carrying out a method according to claim 1.
13. Use of an active stall controlled wind turbine according to
claim 12 in a wind turbine park comprising at least two active
stall controlled wind turbines.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of pending
International patent application PCT/DK2007/000520 filed on Nov.
26, 2007 which designates the United States and claims priority
from Danish patent application PA 2006 01618 filed on Dec. 8, 2006,
the content of which is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The invention relates to a method for damping edgewise
oscillations in one or more blades of a wind turbine, an active
stall controlled wind turbine and use hereof.
BACKGROUND OF THE INVENTION
[0003] A wind turbine known in the art comprises a tapered wind
turbine tower and a wind turbine nacelle positioned on top of the
tower. A wind turbine rotor with a number of wind turbine blades is
connected to the nacelle through a low speed shaft, which extends
out of the nacelle front as illustrated on FIG. 1.
[0004] Oscillations and vibrations of the wind turbine blades are
undesirable in that, they can damage the blades in worse case. In
particular edge-wise oscillations, which are oscillations along the
chord between the trailing edge and the leading edge of the blade,
can damage the blade, in that the blade has little damping towards
this mode of oscillations.
[0005] Furthermore, edgewise oscillations are particularly harmful,
in that they among other things can cause cracks at the root of the
blade or along the trailing edge. In known cases such oscillations
has caused the blade to fail to such a degree, that the blade has
disintegrated from the turbine.
[0006] Both stall and pitch controlled wind turbine are in risk of
being damaged by edge-wise oscillations. The stall controlled
turbine is mostly seeing this problem when operating in high winds
beyond the stall point and the pitch regulated turbine is mostly
seeing this problem in medium wind where sudden wind gusts can
cause the blades to stall momentarily.
[0007] To eliminate harmful oscillations of the blades it is known
to shut down the wind turbine for a period of time, if potentially
damaging edgewise oscillations of the blades is detected. But if
these oscillations are detected often, this method will seriously
reduce the overall output of the wind turbine.
[0008] It is also known to provide the blades with different forms
of mechanical dampers, most often based on the principle of a
spring mounted mass combined with a damping device or they can be
provided with different kinds of liquid dampers.
[0009] An example of a liquid damper is disclosed in WO 99/32789,
where the tips of the blades are provided with a tuned liquid
damper system. A liquid flows freely in a number of cambers placed
as close to the tip of the blade as possible. The chambers have a
specific length, which is adapted to the natural edgewise frequency
of the specific blade type.
[0010] Even though these kinds of frequency specific dampers weigh
less than traditional multi-frequency dampers, they still have the
disadvantage of adding considerable weight to the tip of the blade,
where weight is least desired and under all circumstances it is
undesired to provide anything that can break down in the blades,
both because the inside of the blades can be very difficult to
access and because any extra weight in the blades is undesired.
[0011] An object of the invention is to provide for a wind turbine
comprising means for damping or eliminating edgewise oscillations
in the blades, which do not present the mentioned
disadvantages.
[0012] Furthermore, it is an object of the invention to provide for
a simple and cost-efficient technique for damping or eliminating
edgewise oscillations of one or more blades of a wind turbine.
SUMMARY OF THE INVENTION
[0013] The invention provides for a method for damping edgewise
oscillations in one or more blades of a wind turbine. The method
comprises the steps of [0014] detecting if one or more of the
blades oscillates edgewise during operation of the wind turbine,
and [0015] substantially cyclically generating a pitch angle
difference between at least two of the blades.
[0016] Edgewise oscillations of a blade will result in deflections
of the centre of the hub, on which the blade is attached. These
deflections are synchronous with the oscillations and by cyclically
generating a pitch angle difference between at least two of the
blades the deflections can be disrupted, whereby the oscillations
are dampened.
[0017] Most modern wind turbines are by their nature provided with
the ability to change the pitch angle of the blades and thereby
adjust the blades angle of attack to control the power output of
the rotor or the wind turbine, to protect the blades or the wind
turbine from damaging overloads or other.
[0018] The ability to pitch the wind turbine blades is therefore
already present in most modern wind turbines and by using this
ability to cyclically generating a pitch angle difference between
at least two of the blades when edgewise oscillations are detected
provides for a simple and cost-efficient way of damping edgewise
oscillations of a wind turbine blade.
[0019] It should be emphasised that by the term "during operation"
is to be understood that the wind turbine is producing power to a
utility grid i.e. the rotor of the wind turbine is not stopped and
the rotor is not just idling making the generator only produce
power to sustain the wind turbine itself.
[0020] In an aspect of the invention, said pitch angle difference
is only generated if the size of said edgewise oscillations is
above a predefined level in one or more of said blades.
[0021] Changing the blades pitch angle from their substantially
optimal position power-production-wise, could reduce the power
output of the wind turbine and cyclically pitching one or more of
the blades constantly will also wear the blades pitch mechanism, so
if the size of the edgewise oscillations is only minor and/or
non-damaging, it is advantageous to refrain from pitching the
blades if the size of the oscillations is below a certain
level.
[0022] In an aspect of the invention, the size of said pitch angle
difference is generated substantially directly proportional to the
size of said edgewise oscillations.
[0023] Hereby it is possible to create a simple and efficient
control algorithm which efficiently adapts the generated pitch
angle difference to the size of the oscillations.
[0024] In an aspect of the invention, said pitch angle difference
between at least two of said blades generates an asymmetric load
situation on the hub centre of a rotor to which said blades are
attached.
[0025] By establishing an asymmetric load situation in the
rotor-plane on the hub centre of the rotor it is possible to
deflect the hub centre e.g. making the centre describe an ellipse
during rotation of the rotor. This deflection can disturb the
edgewise motion of the blades and thereby dampen it.
[0026] In an aspect of the invention, said asymmetric load
situation generated by said pitch angle difference substantially
counter-phases an asymmetric load situation generated by said
edgewise oscillations.
[0027] The frequency of the asymmetric load situation generated by
the split pitch can online be tuned to fit the frequency of the
asymmetric load situation generated by the edgewise vibrations and
by making the asymmetric load situation--generated by the pitch
angle difference--substantially counter-phases the asymmetric load
situation--generated by the edgewise oscillations--it is possible
to counteract the edgewise oscillations of the blades and thereby
dampen the oscillations more efficiently.
[0028] In an aspect of the invention, the sum of the thrust
provided by said blades are being substantially maintained when
generating said pitch angle difference between at least two of said
blades.
[0029] At some sites around the world, the wind conditions can
result in that potentially damaging edgewise blade oscillations
occur constantly. It is therefore advantageous that the rotor
thrust is substantially maintained, when carrying out a method for
damping or eliminating these oscillations, in that it hereby is
possible to substantially maintain the wind turbines total power
output even though tower oscillations is dampened.
[0030] In an aspect of the invention, said pitch angle difference
is generated by offsetting the pitch angle of at least a first
blade in a first direction and offsetting the pitch angle of at
least one further blade in the opposite direction of said first
direction.
[0031] By offsetting at least on blade positive and offsetting at
least one further blade negative, the power output of one blade is
substantially increased and the power output of another blade is
substantially reduced, hence the overall power output of the rotor
is substantially maintained.
[0032] In an aspect of the invention, said offset is generated in
addition to the normal pitch angle algorithm controlling the pitch
angle of said blades in relation normal wind turbine control
parameters such to load, power output, wind speed, noise emission,
tower vibrations and/or other.
[0033] By making this change in pitch angle relative--meaning that
it is an extra change aside from the pitch angle change being
preformed to optimise the blades angle to the incoming wind in
relation to power output, load, noise or other--it is possible to
still optimize the blades pitch angle to one or more of these
control parameters even though a cyclically pitch angle difference
is created. This is advantageous in that it hereby is possible to
maintain the power output of the wind turbine or at least reduce
the loss in power output.
[0034] In an aspect of the invention, said edgewise oscillations
are detected by use of one or more oscillation sensors placed in
one or more of said blades.
[0035] By placing the oscillation sensors in the blades it is
possible to obtain much more precise information of the individual
blades edgewise oscillation conditions. This is advantageous in
that it hereby is possible to counteract the motion of the hub
centre more exactly and thereby dampen the oscillations more
efficiently.
[0036] In an aspect of the invention, said edgewise oscillations
are detected by use of one or more oscillation sensors placed in or
at the rotational axis of a rotor on which said blades is
mounted.
[0037] By placing the oscillation sensors in or at the rotational
axis of the rotor it is possible to place the sensors in the
nacelle or in the rotor hub, where the sensors are much easier
accessed.
[0038] In an aspect of the invention, said oscillation sensors are
one or more accelerometers, in that accelerometers are a simple and
cost-efficient means for detecting oscillations.
[0039] In an aspect of the invention, the gain of a control
algorithm controlling the relation between said pitch angle
difference and said edgewise oscillations is increased if the size
of said edgewise oscillations rises above a predefined level.
[0040] When the gain is increased it is possible that the power
output of the wind turbine is reduced, but if the edgewise
oscillations rises above a predefined level the risk of the blades
being damaged is also increased and it is therefore advantageous to
increase the gain to protect the blades.
[0041] In an aspect of the invention, a gain of a control algorithm
is controlling the relation between the time derivatives of said
pitch angle difference and said edgewise oscillations.
[0042] If the gain is controlling the relation between the time
derivatives of the pitch angle difference and the edgewise
oscillations it is possible for the control algorithm to control
the pitch angle difference more accurate in relation to the
edgewise oscillations, particularly regarding fast changes in the
edgewise oscillation level.
[0043] The invention further provides for an active stall
controlled wind turbine comprising control means for carrying out a
method as described above.
[0044] Providing an active stall controlled wind turbine with
control means for carrying out the mentioned method is
advantageous, in that due to the fact that the blades of active
stall controlled wind turbines stalls during normal operation, the
chance of edgewise oscillations occurring is particularly high with
this type of wind turbine. Furthermore, the design of the blades on
active stall controlled wind turbines makes them particularly
vulnerable to edgewise oscillations and it is therefore
particularly advantageously to use this method on an active stall
controlled wind turbine.
[0045] Even further the invention provides for use of an active
stall controlled wind turbine as described above in a wind turbine
park comprising at least two active stall controlled wind
turbines.
[0046] If the wind situation creates edgewise oscillations in the
blades of one wind turbine in a wind turbine park, there is a high
probability that the wind situation will also create edgewise
oscillations in the blades of other wind turbines in the park. If
many wind turbines in a park are shut down substantially
simultaneously due to critical edgewise oscillations of the blades,
it is particularly critical because it is difficult for the power
company to compensate for this sudden large loss in power, and it
is therefore particularly advantageous to use an active stall
controlled wind turbine according to the invention in a wind
turbine park, in that an active stall controlled wind turbine
according to the invention much more often will maintain the power
production and even if it occasionally has to shut down--to prevent
edgewise oscillations from damaging the blade--the risk of several
wind turbines according to the invention in the same wind turbine
park shutting down simultaneously is greatly reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] The invention will be described in the following with
reference to the figures in which
[0048] FIG. 1. illustrates a large modern wind turbine known in the
art, as seen from the front,
[0049] FIG. 2 illustrates a wind turbine blade, as seen from the
front,
[0050] FIG. 3 illustrates an wind turbine comprising blades in
different pitch angles, as seen from the front,
[0051] FIG. 4 illustrates a cross-section of a wind turbine blade
in a non-stall situation, as seen from the root of the blade,
[0052] FIG. 5 illustrates a cross-section of a wind turbine blade
in a stall situation, as seen from the root of the blade,
[0053] FIG. 6 illustrates a cross-section of a wind turbine blade
in a deep stall situation, as seen from the root of the blade,
and
[0054] FIG. 7 illustrates a simplified cross section of a nacelle,
as seen from the side.
DETAILED DESCRIPTION OF THE INVENTION
[0055] FIG. 1 illustrates a modern wind turbine 1, comprising a
tower 2 and a wind turbine nacelle 3 positioned on top of the tower
2. The wind turbine rotor 4, comprising three wind turbine blades
5, is connected to the nacelle 3 through the low speed shaft which
extends out of the nacelle 3 front.
[0056] Each of the blades 5 comprise a tip 8 and a root 9 and at
the root 9 each of the blades 5 are provided with a pitch mechanism
13, enabling that the blades 5 can be rotated individually around
their longitudinal axis.
[0057] FIG. 2 illustrates a wind turbine blade 5, as seen from the
front/pressure side 11. The wind turbine blade 5 comprises a
leading edge 6, a trailing edge 7, a tip 8 and a root 9. A wind
turbine blade 5 known in the art is typically made of a glass fibre
and resin composite reinforced by carbon fibre, carbon fibre
reinforced wood or a combination hereof.
[0058] A wind turbine blade 5 known in the art, has an elastic
centre which is closer to the leading edge 6 than to the trailing
edge 7, at least regarding most parts of the blade 5. If edgewise
oscillations occur at a frequency at or close to the blades first
natural edgewise frequency, especially the trailing edge 7 is
therefore exposed to considerable strain, which under certain
conditions can damaged the blade 5 and result in cracks 10 along
the trailing edge 7.
[0059] FIG. 3 illustrates a wind turbine 1 comprising three blades
5 each positioned in a pitch angle different from the pitch angle
of the two other blades 5, as seen from the front.
[0060] In this embodiment the different pitch angles are cyclically
generated by not offsetting the pitch angle of the first blade 5,
offsetting the pitch angle of the second blade +0.50 and offsetting
the pitch angle of the third blade -0.5.degree.. In another
embodiment the blades 5 could be offset to different pitch angles
or the order or the magnitude of which the blades 5 are pitched
could be different.
[0061] In this embodiment the wind turbine 1 is an active stall
controlled wind turbine 1 but in another embodiment the wind
turbine 1 could be a pitch controlled wind turbine 1 or another
type of wind turbine 1 as long as it comprises means for adjusting
and controlling the pitch angle of the blades 5. The difference
between active stall controlled wind turbines 1 and pitch
controlled wind turbines 1 will be further discussed under FIGS. 4
and 5.
[0062] When certain wind conditions are present--such as the wind
speed is within a specific range, the wind being particularly high
turbulent and/or other, there is a risk of edgewise oscillations in
the blades 5 occurring.
[0063] When edgewise oscillations occur it is usually in a form
which induces an asymmetrical load on the rotor hub 14 centre. This
could e.g. be because only one blade 5 was oscillating, if two
blades 5 where oscillating against and away from each other in
time, if two blades 5 where oscillating in the same direction in
time each at half the amplitude of a third blade 5 oscillating in
the opposite direction or another edgewise oscillating mode which
would inflict an unbalanced load situation in the hub 14 centre.
This asymmetrical load causes the hub 14 centre to deflect in an
elliptical orbit due to summed inertial loading from the
progressive and regressive edgewise blade 5 whirling mode.
[0064] In this embodiment of the invention these edgewise
oscillations of the blades 5 is dampened by cyclically generating a
pitch angle difference between all the blades 5 of the rotor 4 but
in another embodiment it could also be done by cyclically
generating a pitch angle difference between at least two of the
blades 5.
[0065] In this embodiment the pitch angle difference induces an
aerodynamic force in the hub 14 centre which through a control
algorithm in control means 25 is brought to counter-phase with the
velocity of the elliptical orbital describing the hub 14 centre
deflection due to summed inertial loading from the progressive and
regressive edgewise oscillations of the blades 5. In other
words--the forces originating from the edgewise oscillations and
acting on the hub 14 centre is by use of the control means being
counter-phase with forces acting on the hub 14 centre which
originates from cyclically generating a pitch angle difference,
hereby damping or eliminating these oscillations.
[0066] In this embodiment of the invention the wind turbine 1
comprises control means 25 which controls the size of the generated
pitch angle difference i.e. the size of the counter-phased force
acting on the hub 14 centre in direct proportion with the size of
the edgewise oscillations of the blades i.e. the size of the force
generated by the edgewise oscillations acting on the hub 14
centre.
[0067] In another embodiment of the invention the relation between
the input signal (the magnitude of the deflection of the hub 14
centre, the size of the edgewise oscillations or other) and the
output signal (the size of the pitch angle difference) could be
exponential, it could be controlled in steps (if the edgewise
oscillations are within a certain predefined range, the size of the
pitch angle difference is offset a certain predefined size) or
other.
[0068] On active stall controlled wind turbines 1, pitch controlled
wind turbines 1 and other wind turbines 1 comprising pitch
mechanisms 13 the blades 5 can be pitched on the basis of many
different wind turbine control parameters such load, thrust, wind
speed, rotation speed, noise emission, tower vibrations and/or
other. When the pitch angle difference is generated it is by
offsetting the pitch angle of one or more of the blades 5 in
relation to the normal pitch angle algorithm controlling the pitch
angle of the blades 5.
[0069] By cyclically creating this heterogeneous pitch angle
situation (as illustrated in FIG. 3) it is possible to
substantially maintain the overall power output of the rotor 4, in
that the power output of a first blade 5 is unchanged, the power
output of a second blade 5 is slightly increased and the power
output of a third blade 5 is slightly reduced.
[0070] In another embodiment of the invention the pitch angle of
only one blade 5 is offset, leaving the pitch angle of the
remaining blades 5 unchanged during the cyclic attempt to dampen or
eliminate edgewise oscillations of the blades 5.
[0071] In this embodiment of the invention the wind turbine 1
comprise three blades 5 but in another embodiment the wind turbine
1 could comprise another number of blades such as two, four or
more.
[0072] If the wind turbine 1 only comprised two blades 5 only the
pitch angle of one of the blades 5 could be offset to generate the
pitch angle difference or both blades 5 could be offset in opposite
directions.
[0073] If the wind turbine 1 comprised four or more blades the
pitch angle difference could be enabled e.g. by pairing the blades
5 and then offsetting the pitch angles of these pairs in different
directions or e.g. by offsetting the blades' pitch angles in
opposite directions alternately or otherwise.
[0074] In this embodiment of the invention the control means 25
further comprise a dead band or another control method which
ensures that the pitch angle difference is only created when the
edgewise oscillations of the blades 5 are above a certain
predefined level.
[0075] Furthermore, in this embodiment of the invention the control
means 25 also comprise means for increasing the gain of the control
means if the size of the oscillations rises above a certain
predefined level, if the size of the oscillations has not been
dampened below a predefined level within a certain predefined time
or if the size of the oscillations has been above a certain
predefined level for at least a certain predefined time.
[0076] The gain is the part of the control algorithm in the control
means 25 which controls the size of the reaction at a given
oscillation level e.g. by controlling how much the input signal
(the amplitude of the edgewise oscillations) is amplified in the
control algorithm in the control means 25, hereby controlling how
big a counter-phased forced is created in the hub 14 centre by
controlling the pitch angle difference created at a given input
signal.
[0077] An even further way of controlling the dampening process is
as follows: Vibration sensor (accelerometers, strain gauges, etc.)
picks up signals from at least two of the wind turbine blades 5 to
detect the edgewise oscillations status. It is possibly to
decompose the edgewise vibrations into two submodes (progressive
and regressive rotor 4 whirling--modes involving all three blades
5) and to identify the time dependent magnitude of the
corresponding mode amplitudes. Since any edgewise deflection on the
blades 5 can be described by a linear combination of the two
submodes, it is possibly to separate the cyclic split pitch action
i.e. the pitch angle differences to suppress the two submodes, and
to superimpose the corresponding two split pitch demands (onto the
collective pitch demand). The frequencies at which split pitch is
active will be different from each of the two modes (if the blade 5
edgewise mode is strictly in-plane the frequency difference between
the two will be 1P, but that is not necessary so). This control
approach is denoted modal-control, i.e. two parallel PI(D)
controllers perform/control the pitch angle differences (the cyclic
split pitch) to suppress the two edgewise rotor 4 whirling
modes.
[0078] FIG. 4 illustrates a cross-section of a wind turbine blade 5
in a non-stall situation, as seen from the root 9 of the blade
5.
[0079] The blade 5 illustrated in FIG. 4 is a blade 5 on an
ordinary pitch regulated wind turbine 1, shown during normal
operation. In another embodiment it could also be a blade 5 on an
active stall regulated wind turbine 1, operating in low wind or
during start up before the blade 5 starts to stall.
[0080] On a pitch controlled wind turbine 1 the turbines electronic
controller checks the power output of the turbine 1 e.g. several
times per second. When the power output becomes too high, the
controller sends an order to the blade pitch mechanism 13, which
immediately pitches (turns) the rotor blades 5 slightly out of the
wind. Likewise, the blades 5 are turned back into the wind whenever
the wind drops again. During normal operation the blades 5 of a
pitch regulated wind turbine 1 usually only pitch a fraction of a
degree at a time--and the rotor 4 will be turning at the same
time.
[0081] Most known pitch controlled wind turbines 1 do not comprise
detection means 21 for detecting edgewise oscillations of the
blades 5 and do therefore neither comprise active means for damping
or eliminating these vibrations. When providing a pitch controlled
wind turbine 1 with means according to the invention, it is
therefore possible to increase the output of the blades 5, because
it is possible to reduce the margin of safety to stall, in that
means is hereby provided to the wind turbine 1 for damping or
eliminating damaging edgewise oscillations if they should
occur.
[0082] On a pitch controlled wind turbine 1, the controller will
generally pitch the blades 5 slightly every time the wind changes
in order to keep the rotor blades 5 at the optimum angle in order
to maximise output for all wind speeds or at least up to a certain
wind speed such as 25 meters/sec., where the blades 5 are turned
completely out of the wind--making the blade chord C (the line
between the trailing edge 7 and the leading edge 6) substantially
parallel with the wind direction, making the rotor 4 stop rotating
or at least making it idle. Doing this protects the blades 5 from
damaging overload at high wind speeds and this is one of the
reasons that the blades 5 of a pitch controlled wind turbine 1 can
be made relative long and slender, compared to blades 5 of an
active stall regulated wind turbine 1.
[0083] The blades 5 on a pitch controlled wind turbine 1 do usually
not stall during normal operation, in that the blades 5 are pitched
out of the wind before stall can occur. But under certain
circumstances gusts of wind can arise so fast, that the turbines 1
control is not able to react fast enough and for a short period of
time stall can occur. These short stall periods can induce edgewise
oscillations in the blade 5, which potentially can be very
damaging. Particularly if these gusts happen rhythmically at a
frequency at or close to the blades 5 first natural edgewise
frequency the energy of the edgewise oscillations can build up.
[0084] FIG. 5 illustrates a cross-section of a wind turbine blade 5
in a stall situation, as seen from the root 9 of the blade 5.
[0085] The blade 5 illustrated in FIG. 5 is a blade 5 on an active
stall regulated wind turbine 1, shown during normal operation. In
another embodiment it could also be a blade 5 on a pitch regulated
wind turbine 1, illustrated during a sudden gust of wind creating
an undesired stall situation.
[0086] Technically an active stall controlled wind turbine 1
resembles a pitch controlled wind turbine 1, in that they both have
pitchable blades, and in order to get a reasonably large torque
(turning force) at low wind speeds, the active stall controlled
wind turbine 1 will usually be programmed to pitch the blades 5
much like a pitch controlled wind turbine 1 at low wind speeds.
When the active stall controlled wind turbine 1 reaches its rated
power, however, one will notice an important difference from the
pitch controlled wind turbines 1: If the generator is about to be
overloaded, the active stall controlled wind turbine 1 will pitch
its blades 5 in the opposite direction from what a pitch controlled
wind turbine 1 does. In other words, it will increase the angle of
attack of the rotor blades 5 in order to make the blades 5 go into
a deeper stall, thus wasting the excess energy in the wind. At high
wind speeds the blades 5 of an active stall controlled wind turbine
1 will therefore have to be able to withstand a much higher extreme
load than blades 5 of a pitch controlled wind turbine 1, both just
to keep the blades 5 from breaking and to keep the blades 5 from
bending so much that there is a risk of them hitting the tower 2.
The blades 5 of an active stall controlled wind turbine 1 are
therefore made more rugged and heavy than blades 5 of a pitch
controlled wind turbine 1.
[0087] Furthermore, stall creates noise and to reduce the noise
emission from the active stall controlled wind turbine 1 the rotor
4 rotates slower than the rotor 4 of a pitch controlled wind
turbine 1. The blades 5 of an active stall controlled wind turbine
1 therefore have to be bigger and wider to be able to utilize the
energy of the wind efficiently.
[0088] One of the advantages of active stall controlled wind
turbines 1 compared to passive stall controlled wind turbines 1 is
that the power output can be controlled more accurately, so as to
avoid overshooting the rated power of the wind turbine 1 at the
beginning of a gust of wind. Another advantage is that active stall
controlled wind turbines 1 can be run almost exactly at rated power
at all high wind speeds at least up to a certain maximum wind
speed. A normal passive stall controlled wind turbine 1 will
usually have a drop in the electrical power output for higher wind
speeds, as the rotor blades 5 go into deeper stall.
[0089] FIG. 6 illustrates a cross-section of a wind turbine blade 5
in a deep stall situation, as seen from the root 9 of the blade
5.
[0090] The blade 5 illustrated in FIG. 6 is a blade 5 on an active
stall regulated wind turbine 1, shown during operation at very high
wind speeds.
[0091] In this embodiment the blade 5 is pitched into the wind
making it stall and substantially lose all the energy of the wind
to protect the wind turbine 1 from damaging overloads.
[0092] FIG. 7 illustrates a simplified cross section of a nacelle 3
of an active stall regulated wind turbine 1, as seen from the side.
Nacelles 3 exists in a multitude of variations and configurations
but in most cases the drive train in the nacelle 3 comprise one or
more of the following components: a gear 15, a coupling (not
shown), some sort of breaking system 16 and a generator 17. A
nacelle 3 of a modern wind turbine 1 can also include a converter
18 (also called an inverter) and additional peripheral equipment
such as further power handling equipment, control cabinets,
hydraulic systems, cooling systems and more.
[0093] The weight of the entire nacelle 3 including the nacelle
components 15, 16, 17, 18 is carried by a strengthening structure
19. The components 15, 16, 17, 18 are usually placed on and/or
connected to this common load carrying structure 19. In this
simplified embodiment the strengthening structure 19 only extends
along the bottom of the nacelle 3 e.g. in form of a bed frame 20 to
which some or all the components 15, 16, 17, 18 are connected. In
another embodiment the strengthening structure 19 could comprise a
gear bell transferring the load of the rotor 4 to the tower 2, or
the load carrying structure 19 could comprise several
interconnected parts such as latticework.
[0094] In this embodiment of the invention the drive train is
established in an angle in relation to a horizontal plane. The
drive train is for among other reasons angled to enable that the
rotor 4 can be angled correspondingly e.g. to ensure that the
blades 5 do not hit the tower 2, to compensate for the differences
in wind speed at the top and bottom of the rotor 4 and other.
[0095] In this embodiment of the invention oscillation sensors 21
are placed in each of the blades 5 to detect if the blades 5
oscillates edgewise. In that the amplitude of edgewise oscillations
of a blade 5 will increase with the distance from the root 9 of the
blade 5 the oscillation sensors 21 are in this embodiment of the
invention accelerometers placed inside the blades 5 at a given
distance from the root 9 of the blade.
[0096] In another embodiment of the invention the oscillation
sensors 21 could be other types of sensors than accelerometers
22--such as microphones, strain-gauges, optical fibres or other, it
or they could be placed differently in the blades 5 or the
oscillation sensors 21 could be placed outside the blades 5 such as
in or at the rotational axis 26 of a rotor 4 e.g. in the hub 14
centre or in the nacelle 3.
[0097] The oscillation sensors 21 are in this embodiment of the
invention connected to control means 25. If edgewise oscillations
of the blades 5 are detected or if edgewise oscillations above a
certain level are detected, the control means 25 can initiate that
one or more of the blades 5 are cyclically pitched.
[0098] As previously explained the blades 5 of an active stall
regulated wind turbine 1 or a pitch regulated wind turbine are
provided with a pitch mechanism 13. In the illustrated embodiment
the blades 5 of the wind turbine 1 are connected to the hub 14
through pitch bearings 23, enabling that the blades can rotate
around their longitudinal axis.
[0099] In this embodiment the pitch mechanism 13 comprise means for
rotating the blades in the form of linear actuators 24 connected to
the hub 14 and the respective blades 5. In a preferred embodiment
the linear actuators 24 are hydraulic cylinders. In another
embodiment the pitch mechanism 13 could comprise stepper motors or
other means for rotating the blades 5.
[0100] In this embodiment the control means 25 is placed in the hub
14 but in a more preferred embodiment the control means 25 would be
placed in the nacelle 3, in the tower 2, in a neighboring house or
elsewhere e.g. at the same location as the general pitch control
means (not shown) for the controlling the pitch in relation to
load, power or other or even integrated in these general pitch
control means.
[0101] In this embodiment the control means 25 are connected to the
linear actuators 24 for controlling the pitch angle of the blades 5
in response to the measurements of the oscillation sensors 21.
[0102] If the edgewise oscillations has not dropped below a
predetermined level within a predetermined period of time the
control means 25 could comprise means for sending of an alarm
and/or sending a signal initiating that the wind turbine was shut
down. Likewise, if the edgewise oscillations continues to grow in
size--even though the control means 25 cyclically sends a signal to
counteract the oscillations by cyclically creating a pitch angle
difference--an alarm signal and/or a signal to stop the wind
turbine can be send.
[0103] The invention has been exemplified above with reference to
specific examples of wind turbines 1, oscillation sensors 21,
methods for damping edgewise oscillations and other. However, it
should be understood that the invention is not limited to the
particular examples described above but may be designed and altered
in a multitude of varieties within the scope of the invention as
specified in the claims.
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